Lewis and Clark County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 63,395. Its county seat is Helena, the state capital. The numerical designation for Lewis and Clark County (used in the issuance of the state's license plates) is 5. The county was established in 1865 as Edgerton County, and was renamed "Lewis and Clark County" two years later. The present name was given in honor of explorers Lewis and Clark.